[native] Make native build on Windows

Summary:
Part of ENG-2457.
We want to be able to build the desktop app on Windows but currently, the postinstall script of native workspace doesn't work on Windows:

• it uses the bash syntax (and on windows postinstall runs under cmd.exe)
• there are some problems with paths when it comes to patch-package because of some symlink weirdness in node_modules

This diff moves the postinstall logic into a separate script file and runs it with bash explicitly. If we are on windows we skip everything (we are only going to be building the desktop app).
This will work on windows provided there's a bash executable in PATH, thankfully that's true for windows github runners (bash is included in the git for windows).

Additionally, we skip pod install if we aren't on macOS instead of using || true which hides errors.

Test Plan:

• yarn cleaninstall on macOS laptop works
• CI on phab works
• yarn cleaninstall on Windows laptop and on GitHub Windows Runner (in a forked repo) works

Reviewers: atul, varun, jon, O2 Blocking Reviewers, tomek

Reviewed By: jon, O2 Blocking Reviewers, tomek

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D6142